1. Field of the Invention
This invention generally relates to methods for the synthesis of nanostructured materials. In particular, this invention relates to chemical methods for the introduction of a grain growth inhibitor and/or alloy additions into nanostructured materials, resulting in materials having controlled microstructure and morphology. The obtained materials exhibit superior properties, including improved fracture toughness, hardness, and wear-, corrosion-, and erosion-resistance.
2. Brief Description of the Prior Art
For decades, materials with fine-scale microstructures have been recognized to exhibit unusual and technologically attractive properties. Currently, interest is growing in a new class of materials that are composed of very fine grains or particles having dimensions in the range of 1-100 nanometers (nm), known as "nanostructured" materials. A feature of nanostructured materials is the high fraction of atoms (up to 50%) that reside at grain or particle boundaries. Nanostructured materials thus have substantially different, very often superior chemical and physical properties compared to conventional, micron-sized counterparts having the same composition. Thus considerable advantages accrue from the substitution of nanostructured materials for conventional microstructured materials in a wide range of applications. As set forth in the examples in Nanostructured Materials, Vol. 1, 1992, p.1 et seq., these include superior strength, improved fracture toughness and hardness in martensitic steels, reduced sintering temperature for consolidation and the onset of superplasticity in nanostructured ceramics, improved ductility in ceramic/metal composites (cermets), reduced thermal conductivity in thermal barrier materials, and outstanding catalytic and electrochemical properties. Nanostructured metal alloys and metal carbides, in particular, are expected to have superior properties, including improved fracture toughness, hardness, and wear-, corrosion-, and erosion-resistance. The ability to synthesize and optimize the pore structure and packing density of nanostructured materials at the atomic level is a powerful tool for obtaining new classes of nanostructured materials. These new classes of nanostructured materials, together with designed multifunctional coatings, present unprecedented opportunities for advances in material properties and performance for a broad range of engineering applications.
Inframat Corp. has made significant progress in the field of nanostructured materials, including in the synthesis of nanostructured metal powders by the organic solution reaction (OSR) method, aqueous solution reaction (ASR) method, and in advanced chemical processing of oxides and hydroxides materials for structural, battery and fuel cell applications. Examples of materials produced from these methods include nanostructured alloys of Ni/Cr, nanostructured NiCr/Cr.sub.3 C.sub.2 composites, nanostructured yttria-stabilized ZrO.sub.2, nanofibrous MnO.sub.2, and Ni(OH).sub.2. Inframat has further developed technologies for manufacturing nanostructured materials in bulk quantities as disclosed in "Nanostructured Oxide and Hydroxide Materials and Methods of Synthesis Therefor," which is the subject of pending U.S. and foreign applications (including U.S. Ser. No. 08/971,817 filed Nov. 17, 1997), as well as technologies for the thermal spray of nanostructured feeds including nanostructured WC--Co composite, as disclosed in "Nanostructured Feeds for Thermal Spray Systems, Method of Manufacture, and Coating Formed Therefrom" also the subject of pending U.S. and foreign patent applications (including U.S. Ser. No. 09/019,061, filed Feb. 5, 1998), both U.S. patent applications being incorporated herein in their entirety. Chemical syntheses of nanostructured metals, ceramics, and composites using OSR and ASR methods have also been previously described by Xiao and Strutt in "Nanostructured Metals, Metal Alloys, Metal Carbides and Metal Alloy Carbides and Chemical Synthesis Thereof," U.S. Ser. No. 08/989,000, filed Dec. 5, 1996, incorporated herein by reference, as well as in "Synthesis and Processing of Nanostructured Ni/Cr and Ni--Cr.sub.3 C.sub.2 Via an Organic Solution Method," Nanostructured Mater. Vol. 7 (1996) pp. 857-871 and in "Synthesis of Si(C,N) Nanostructured Powders From an Organometallic Aerosol Using a Hot-Wax Reactor," J. Mater. Sci. Vol. 28 (1993), pp. 1334-1340.
The OSR and ASR methods employ a step-wise process generally comprising (1) preparation of an organic (OSR) or aqueous (ASR) solution of mixed metal halides; (2) reaction of the reactants via spray atomization to produce a nanostructured precipitate; and (3) washing and filtering of the precipitate. The precipitate is often then heat treated, and/or subjected to gas phase carburization under either controlled carbon/oxygen activity conditions (to form the desired dispersion of carbide phases in a metallic matrix phase), or under controlled nitrogen/hydrogen activity conditions to form nanostructured nitrides. This procedure has been used to synthesize various nanostructured compositions, including nanostructured NiCr/Cr.sub.3 C.sub.2 powders for use in thermal spraying of corrosion resistant hard coatings. An advanced chemical processing method combines the ASR and OSR methods with spray atomization and ultrasonic agitation.
Another approach to the synthesis of nanostructured materials is the inert gas condensation (IGC) method. As described in "Materials with Ultrafme Microstructures: Retrospective and Perspectives", Nanostructured Materials Vol. 1, pp. 1-19, Gleiter originally used this method to produce nanostructured metal and ceramics clusters. The method was later extensively used by Siegel to produce nanostructured TiO.sub.2 and other systems, as described in "Creating Nanophase Materials", Scientific American Vol. 275 (1996), pp.74-79. This method is the most versatile process in use today for synthesizing experimental quantities of nanostructured metals and ceramic powders. The IGC method uses an evaporative sources of metals, which are then convectively transported and collected on a cold substrate. Ceramic particles must be obtained by initially vaporizing the metal source, followed by a slow oxidation process. A feature of this method is the ability to generate loosely agglomerated nanostructured powders, which are sinterable at low temperatures.
One other method for the synthesis of nanostructured materials is chemical vapor condensation (CVC). CVC is described by Kear et al. in "Chemical Vapor Synthesis of Nanostructured Ceramics" in Molecularly Designed Ultra fine/Nanostructured Materials in MRS Symp. Proc. Vol. 351 (1994), pp. 363-368. In CVC, the reaction vessel is similar to that used the IGC method, but instead of using an evaporative source, a hot-wall tubular reactor is used to decompose a precursor/carrier gas to form a continuous stream of clusters of nanoparticles exiting the reactor tube. These clusters are then rapidly expanded out to the main reaction chamber, and collected on a liquid nitrogen cooled substrate. The CVC method has been used primarily with chemical precursors or commercially available precursors. Kear describes the production of nanostructured SiC.sub.x N.sub.y and oxides from hexamethyldisilazane.
Finally, a thermochemical conversion method for producing nanostructured WC--Co has been disclosed by Kear in "Synthesis and Processing of Nanophase WC--Co Composite" Mater. Sci. Techn. Vol. 6 (1990), p. 953. In this method, aqueous solutions containing tungsten and cobalt precursors are spray-dried to form an intermediate precursor at temperatures of about 150 to 300.degree. C. This intermediate precursor is a mixture of amorphous tungsten oxide and cobalt in the form of a spherical hollow shell having a diameter of about 50 microns and a wall thickness of about 10 microns. Nanostructured WC--Co is then obtained by the carburization of this precursor powder at 800-900.degree. C. in a carbon monoxide/carbon dioxide mixture. The synthesis of nanostructured WC/Co using this technique has described in several patents by McCandlish et al., including "Multiphase Composite Particle," U.S. Pat. No. 4,851,041, and "Carbothermic Reaction Process for Making Nanophase WC--Co," U.S. Pat. No. 5,230,729. Synthesis of nanostructured WC/Co is of particular interest to industry, as is it is presently the material of choice for cutting tool, drill bit, and wear applications.
A major drawback of the above-described nanostructured materials, and in particular nanostructured WC/Co, is the tendency of the produced materials to undergo grain growth during sintering or use of the nanostructured component, especially at high temperature. For example, the tungsten carbide grains of the as-synthesized nanostructured WC--Co have diameters of about 30 mn. However, during liquid phase sintering, the tungsten carbide grains grow rapidly to diameters of several microns in a relatively short time, e.g., a few minutes. After exhaustive research and engineering evaluations it has been concluded that VC and/or Cr.sub.3 C.sub.2 are the most effective carbide phases of the very large range of materials evaluated over the years.
Vanadium carbide, for example, has been employed by Nanodyne, Inc. (Brunswick, N.J.) to prevent this disadvantageous grain growth, as described in Nanodyne's product specification for the product sold under the tradename Nanocarb.RTM.. In this process, micron-sized vanadium carbide powders are blended into the WC/Co powder composite via mechanical mixing, followed by shape-formation and sintering into bulk components. The use of vanadium carbide is effective to prevent some degree of grain growth, as the final grain size of the consolidated bulk piece is in the sub-micron range, e.g., 0.2-0.8 microns. The major drawback of this grain growth technique is the non-homogeneous mixing of the VC within the WC/Co composite materials as well as the difficulty of sintering kinetics, resulting in non-homogeneous bulk material properties, or increased cost for sintering procedure.
In an attempt to solve the above problem, Nanodyne currently employs a chemical precipitation technique in which a vanadium salt is introduced chemically at the start of the materials synthesis. Although use of this technique overcomes the problem of non-homogeneous mixing, it produces vanadium oxide instead of vanadium carbide. It is well known that any oxide material that presented in the WC/Co system is detrimental to the material. The introduction of vanadium oxide into the WC/Co system not only creates sintering difficulties, but also requires an extra carbon source in the WC/Co powder for the conversion of vanadium oxide into vanadium carbide at extremely high temperatures, e.g., 1450.degree. C. In many cases the extra carbon source embrittles the consolidated materials.
Prior incorporation of boron compounds into fine-grained materials has been described in "Synthesis of AlN/BN Composite Materials" by Xiao and Srutt, in J. Am. Ceram. Soc., Vol. 76, p. 987 (1993), which discloses synthesis from a composite precursor comprising aluminum, boron, and nitrogen. The boron nitride polymer is formed by bubbling ammonia into an aqueous solution of boric acid and urea.
Scoville et al., in "Molecularly Designed Ultrafine/Nanostructured Materials", ed. by K. E. Gonsalves, D. M. Chow, T. D. Xiao, and R. Cammarato, MRS Symp. Vol. 351, p. 431 (1994) describe a method for incorporation of BN nanostructured particles into a germanium crystal lattice to significantly reduce thermal conductivity. In this method, micron-sized BN and Si/Ge powders are blended together and evaporated using a plasma torch to form mixtures, which are then condensed into a composite of BN/Si/Ge. Heat treatment results in large crystals (&gt;1 micron) of Si/Ge wherein discrete 2-10 nm BN grains are trapped inside the large crystals.
Boron nitride has also been incorporated into a conventional (larger than 100 nm grain size) titanium diboride system with WC/Co additives as disclosed in U.S. Pat. No. 5,632,941 to Mehrotra et al. BN is incorporated in the form of a powder. U.S. Pat. No. 4,713,123 further disclose use of BN as a grain growth inhibitor in conventional (larger than 100 nm grain size) silicon steel. However, when the quantity of BN is too large, it is becomes difficult to grow the secondary recrystallized grain with {110}&lt;001 &gt;orientation, so that the amount is limited to a range of 0.0003-0.02%. Addition of boron to silicon steel in the form of ferroboron, followed by nitridation of the steel to provide nitrogen results in the formation of slight amounts of boron or BN, which may inhibit grain boundary migration, as reported by Grenoble in IEEE Trans. Mag., May 13th, p. 1427 (1977), and Fiedler in IEEE Trans. Mag. May 13th p. 1433 (1977). None of these references disclose use of a BN polymer as a grain growth inhibitor precursor.
Accordingly, there remains a need in the art for methods of inhibiting grain growth during the processing of as-synthesized nanostructured materials and nanostructured material intermediates, especially a method applicable to a wide range of compositions.